Synthesis of Rhodium, Iridium, and Palladium Tetranuclear Complexes Directed by...

Synthesis of Rhodium, Iridium, and Palladium Tetranuclear Complexes Directed by2,6-Dimercaptopyridine. X-ray Crystal Structure of [Rh 4(µ-PyS2)2(cod)4](cod ) 1,5-Cyclooctadiene)

Jesus J. Perez-Torrente, Miguel A. Casado, Miguel A. Ciriano,* Fernando J. Lahoz, andLuis A. Oro*

Departamento de Quı´mica Inorganica, Instituto de Ciencia de Materiales de Arago´n, Universidad deZaragoza-CSIC, 50009 Zaragoza, Spain

ReceiVed July 7, 1995X

Tetranuclear diolefin complexes of the general formula [M4(µ4-PyS2)2(diolefin)4] [M ) Rh, diolefin )1,5-cyclooctadiene (cod) (1), 2,5-norbornadiene (nbd) (2), tetrafluorobenzobarrelene (tfbb) (3); M ) Ir, diolefin) cod (4), PyS2 ) 2,6-pyridinedithiolate) are prepared in high yield by reaction of the appropriate complex[{M(µ-Cl)(diolefin)}2] with the salt Li2PyS2 generated “in situ”. This method is also used to prepare [Pd4(µ-PyS2)2(allyl)4] (5). Alternative syntheses for these complexes are also described. The structure of1 wasconclusively determined by a single-crystal X-ray analysis. Complex1 crystallizes in the monoclinic system,space groupC2/c, with a) 10.252(1) Å,b) 17.023(2) Å,c) 23.114(3) Å,â ) 99.50(1)°, andZ) 4. Refinementby full matrix least-squares gave finalR ) 0.028 andRw ) 0.024. Complex1 is tetranuclear with two S,N,S-tridentate 2,6-dimercaptopyridine ligands bridging all of the four metallic centers and presents a crystallographicallyimposedC2 symmetry relating two “Rh2(µ4-PyS2)(cod)2” moieties. The two S atoms of each bridging ligandexhibit different coordination modes; while one is bonded to one metal, the second one is coordinated to twodifferent rhodium centers. The shortest Rh‚‚‚Rh separation is 3.1435(5) Å. Carbonylation of the rhodium diolefincomplexes under atmospheric pressure gives [Rh4(µ4-PyS2)2(CO)8] (6) which maintains the molecular frameworkof 1. Further reaction of the carbonyl complex with PPh3 gives [Rh4(µ-PyS2)2(CO)4(PPh3)4] (7), but this complexis prepared more conveniently by reaction of Li2PyS2 with [{Rh(µ-Cl)(CO)(PPh3)}2]. The replacement of CO byPPh3 is not selective, and this complex exists in solution as a mixture of three isomers due to the relative positionof the PPh3 groups. The diolefinic and carbonyl complexes are fluxional. Variable temperature1H and13C{1H}spectra associated with H,H-COSY experiments led to the assignment of the olefinic resonances and the conclusionthat the two diolefins at the inner part of the complexes are rigid, while the two external ones undergo the fluxionalbehavior due to an inversion at the terminal sulfur donor atoms. This is also the origin of the fluxionality of thecarbonyl complex. Deprotonation of Py(SH)2 with [Rh(acac)(cod)] (acac) acetylacetonate) can be carried outstepwise, giving the dinuclear complex [Rh2(µ-PyS2H)2(cod)2] (8), and later the tetranuclear complex1. Thismethod to synthesize heterotetranuclear complexes by the addition of either [Ir(acac)(cod)] or [{Ir(µ-OMe)-(cod)}2] to the isolated dinuclear rhodium complex (8) has been shown to be nonselective, giving a mixture oftetranuclear complexes with the [Rh3Ir] 4+, [Rh2Ir2]4+, and [RhIr3]4+ cores. The rhodium complexes undergo tworeversible one-electron oxidations at a platinum bead electrode in dichloromethane separated by approximately0.4 V at potentialsE° in the ranges 0.0-0.4 and 0.4-0.8 V. The electrochemical behavior of the iridium complexis more complicated, undergoing two similar one-electron oxidations followed by a chemical reaction.

Introduction

The synthesis, reactivity, and structural characterization ofdi- and polynuclear complexes has received considerableattention due to their potential for novel stoichiometric andcatalytic reactions.1 Bidentate bridging ligands such as bis-(diphenylphosphino)methane (dppm) and 2-(diphenylphosphi-no)pyridine (A) (Chart 1) that contain a single bridgehead atom,have proven to be very effective in stabilizing moleculescontaining single or even multiple metal-metal bonds2 due tothe flexibility to adapt both to variations of metal-metal

separation and to the coordination geometries about the twometal atoms.3,4

Going from dinuclear to polynuclear complexes generallyrequires the development of new synthetic strategies in which

X Abstract published inAdVance ACS Abstracts,February 15, 1996.(1) (a)The Chemistry of Metal Cluster Complexes; Shriver, D. F., Kaesz,

H. D., Adams, R. D., Eds.; VCH: New York, 1990. (b)Compre-hensiVe Organometallic Chemistry; Abel, E. W., Stone, F. G. A.,Wilkinson, G., Eds.; Pergamon: Oxford, England, 1982. (c)Homo-geneous Catalysis with Metal Phosphine Complexes; Pignolet, L. H.,Ed.; Plenum Press: New York, 1983. (d) Challoner, P. A.; Esteruelas,M. A.; Joo, F.; Oro, L. A. Homogeneous Hydrogenation; Kluwer:Dordretcht, The Netherlands, 1994.

(2) Cotton, F. A.; Walton, R. A.Multiple Bonds between Metal Atoms;Wiley: New York, 1993.

(3) (a) Puddephatt, R. J.J. Chem. Soc. ReV. 1983, 99. (b) Chaudret, B.;Delavaux, B.; Poilblanc, R.Coord. Chem. ReV. 1988, 86, 191.

(4) Newkome, G. R.Chem. ReV. 1993, 93, 2067. Arena, C. G.; Ciani,G.; Drommi, D.; Faraone, F.; Proserpio, D. M.; Rotondo, E.J.Organomet. Chem. 1994, 484, 71, and references cited therein.

Chart 1

1782 Inorg. Chem.1996,35, 1782-1791

0020-1669/96/1335-1782$12.00/0 © 1996 American Chemical Society

the design of the bridging ligands plays an important role: anincrease of the number of donor atoms in the bridging ligandsand the presence of several lone electron pairs (O or S forexample) are useful alternatives.5 In addition, the frameworkin which the donor atoms are placed should be carefully chosento produce small “bites” and to avoid undesired coordinationmodes. Ligands of these characteristics are, for example, bis-(diphenylphosphinomethyl)phenylphosphine (dpmp),6 bis(diphen-ylphosphinomethyl)phenylarsine (dpam),7 and 2,6-bis(diphen-ylphosphino)pyridine4a,8 (B), which give rise to controlledsynthesis of homo- and heterotrinuclear and tetranuclear com-plexes.8 Other attempts to obtain complexes of high nuclearityby using polydentate ligands such as 7-diphenylphosphino-2,4-dimethyl-1,8-naphthyridine9 and 2-mercapto-7-methyl-1,8-naph-thyridine10 have met little success so far, since only dinuclearcomplexes have been isolated. For synthetical purposes thebridging ligands should expand several metal atoms, as foundin tetranuclear Mo-Pd complexes of 6-(diphenylphosphino)-2-pyridonate.11

We have shown that anionic bidentate ligands having a smallbite and a structural donor unit N-C-S, like pyridine-2-thiolate(C) or benzothiazole-2-thiolate, are useful in building homo-and hetero-dinuclear complexes.5e,12 Some of them behave aseight-membered metallomacrocycles, where a third metallicfragment can be trapped giving rise to homo- and hetero-trinuclear aggregates having the framework [M3(µ-NCS)2] ofpredetermined structure in a controlled way.13 In this context,2,6-dimercaptopyridine (D) is a new system which has not beeninvestigated previously. The doubly thiolate functionalizedpyridine ligand should combine the features shown by other2,6-difunctionalized pyridine derivatives and those of the thione

or thiolate ligands. Therefore, it may interact with transitionmetals in a particularly, attractive manner, with regard to theability to build polynuclear aggregates. We report here thesynthesis and structure of new tetranuclear complexes ofrhodium(I), iridium(I), and palladium(II) supported by 2,6-dimercaptopyridine (D) and their electrochemical properties.

Experimental Section

General. All manipulations were performed under a dry nitrogenatmosphere using Schlenk-tube techniques. Solvents were dried bystandard methods and distilled under nitrogen immediately prior to use.Standard literature procedures were used to prepare the starting materials[{Rh(µ-Cl)(diolefin)}2] (diolefin ) cod,14 nbd15), [{Rh(µ-Cl)(CO)-(PPh3)}2],16 and [{Pd(µ-Cl)(allyl)}2].17 The acetylacetonate complexes[Ir(acac)(cod)] and [Rh(acac)(CO)(PPh3)] were prepared following theprocedure described for [Rh(acac)(cod)],18 and the complexes [{Rh-(µ-OMe)(cod)}2] and [{Ir(µ-OMe)(cod)}2] were prepared according tothe method reported for [{Rh(µ-OMe)(tfbb)}2].19 AgBF4 was purchasedfrom Fluka Chemicals and used as received. 2,6-Dimercaptopyridine[Py(SH)2] was synthesized as described by Pappalardo et al.20 usingthe modified method of Vo¨gtle and Effler.21 The NMR spectra of 2,6-dimercaptopyridine are as follows.1H NMR (toluene-d8, 215 K): δ15.61 (s, 1H, SH), 9.43 (s, 1H, NH), 6.75 (d, 1H, 8.2 Hz), 6.02 (dd,1H, 8.2 and 7.2 Hz), 5.89 (d, 1H, 7.2 Hz).1H NMR spectrum (CDCl3,293 K): δ 9.56 (br, 2H, SH), 7.25 (t, 8.0 Hz, 1H), 6.95 (d, 2H, 8.0Hz). 13C{1H} NMR spectrum (CDCl3, 328 K): δ 160.8 (CS), 137.8,121.8 (2C).Physical Measurements.1H, 13C{1H}, and31P{1H} NMR spectra

were recorded on Varian UNITY 300 and Bruker ARX 300 spectrom-eters operating at 299.95 and 300.13; 75.42 and 75.47; 121.42 and121.49 MHz, respectively. Chemical shifts are reported in parts permillion and referenced to Me4Si using the signal of the deuteratedsolvent (1H and 13C) and 85% H3PO4 (31P) as external reference,respectively. IR spectra were recorded on a Perkin-Elmer 783spectrometer using Nujol mulls between polyethylene sheets or insolution in a cell with NaCl windows. Elemental analyses wereperformed with a Perkin-Elmer 240-C microanalyzer. Conductivitieswere measured inca. 5 × 10-4 M dichloromethane solutions using aPhilips PW 9501/01 conductimeter. Molecular weights were deter-mined with a Knauer osmometer using chloroform solutions of thecomplexes. Mass spectra were recorded in a VG Autospec double-focusing mass spectrometer operating in the FAB+ mode. Ions wereproduced with the standard Cs+ gun atca. 30 kV; 3-nitrobenzyl alcohol(NBA) was used as matrix.Cyclic voltammetric experiments were performed with a EG&G

PARC Model 273 potentiostat/galvanostat. A three-electrode glass cellconsisting of a platinum-disk working electrode, a platinum-wireauxiliary electrode, and a standard calomel reference electrode (SCE)was used. Linear voltamperometry was performed by using a rotatingplatinum electrode (RDE) as the working electrode. Tetra-n-butylam-monium hexafluorophosphate (TBAH) was employed as supportingelectrolyte. Electrochemical experiments were carried out undernitrogen inca. 5× 10-4 M dichloromethane solutions of the complexesand 0.1 M in TBAH. The [Fe(η-C5H5)2]+/[Fe(η-C5H5)2] couple isobserved at+0.47 V under these experimental conditions.Preparation of the Complexes. [Rh4(µ-PyS2)2(cod)4] (1). Method

A. To a solution of [{Rh(µ-OMe)(cod)}2] (0.100 g, 0.206 mmol) indichloromethane (15 mL), solid 2,6-dimercaptopyridine (0.029 g, 0.206mmol) was added. The mixture was stirred for 30 min to give a deepred solution. Concentration of the solution under vacuum to 1 mLand addition of diethyl ether (5 mL) gave an orange microcrystalline

(5) (a) Dunbar, K. R.; Matonic, J. H.; Saharan, V. P.Inorg. Chem. 1994,33, 25. (b) Cayton, R. H.; Chisholm, M. H.; Huffman, J. C.;Lobkovsky, E. B.;Angew. Chem., Int. Ed. Engl. 1991, 30, 862. (c)Cayton, R. H.; Chisholm, M. H.; Huffman, J. C.; Lobkovsky, E. B.J.Am. Chem. Soc. 1991, 103, 8709. (d) Ciriano, M. A.; Villarroya, B.E.; Oro, L. A. Inorg. Chim. Acta 1986, 120, 43. (e) Oro, L. A.;Ciriano, M. A.; Viguri, F.; Tiripicchio, A.; Tiripicchio-Camellini, M.New J. Chem. 1986, 10, 75.

(6) (a) Balch, A. L.; Linehan, J. C.; Olmstead, M. M.Inorg. Chem. 1986,25, 3937. (b) Balch, A. L.; Fosset, L. A.; Guimerans, R. R.; Olmstead,M. M. Organometallics1985, 4, 781. (c) Balch, A. L.; Guimerans,R. R.; Olmstead, M. M.J. Organomet. Chem. 1984, 268, C38. (d)Olmstead, M. M.; Guimerans, R. R.; Balch, A. L.Inorg. Chem. 1983,22, 2473.

(7) (a) Balch, A. L.; Nagle, J. K.; Oram, D. E.; Reedy, P. E., Jr.J. Am.Chem. Soc. 1988, 110, 454. (b) Bailey, D. A.; Balch, A. L.; Fosset,L. A.; Olmstead, M. M.; Reedy, P. E., Jr.Inorg. Chem. 1987, 26,2413. (c) Balch, A. L.; Oram, D. E.; Reedy, P. E., Jr.Inorg. Chem.1987, 26, 1836. (d) Balch, A. L.; Nagle, J. K.; Olmstead, M. M.;Reedy, P. E., Jr.J. Am. Chem. Soc. 1987, 109, 4123. (e) Balch, A.L.; Fosset, L. A.; Olmstead, M. M.; Oram, D. E.; Reedy, P. E., Jr.J.Am. Chem. Soc. 1985, 107, 5272.

(8) Balch, A. L.Prog. Inorg. Chem. 1994, 41, 239.(9) Lo Schiavo, S.; Grassi, M.; De Munno, G.; Nicolo, F.; Tresoldi, G.

Inorg. Chim. Acta1994, 216, 209.(10) Sheldrick, W. S.; Mintert, M.Inorg. Chim. Acta1994, 219, 23.(11) Mashima, K.; Nakano, H.; Nakamura, A.J. Am. Chem. Soc. 1993,

115, 11632.(12) (a) Ciriano, M. A.; Pe´rez-Torrente, J. J.; Lahoz, F. J.; Oro, L. A.Inorg.

Chem. 1992, 31, 969. (b) Yap, G. P. A.; Jensen, C. M.Inorg. Chem.1992, 31, 4823. (c) Reynolds, J. G.; Sendlinger, S. C.; Murray, A.M.; Huffman, J. C.; Christou, G.Angew. Chem., Int. Ed. Engl. 1992,31, 1253. (d) Yamamoto, J. H.; Yoshida, W.; Jensen, C. M.Inorg.Chem. 1991, 30, 1353. (e) Ciriano, M. A.; Viguri, F.; Pe´rez-Torrente,J. J.; Lahoz, F. J.; Oro, L. A.; Tiripicchio, A.; Tiripicchio-Camellini,M. J. Chem. Soc., Dalton Trans. 1989, 25.

(13) (a) Ciriano, M. A.; Pe´rez-Torrente, J. J.; Oro, L. A.; Tiripicchio, A.;Tiripicchio-Camellini, M. J. Chem. Soc., Dalton Trans. 1991, 225.(b) Ciriano, M. A.; Pe´rez-Torrente, J. J.; Viguri, F.; Lahoz, F. J.; Oro,L. A.; Tiripicchio, A.; Tiripicchio-Camellini, M.J.Chem. Soc.,DaltonTrans. 1990, 1493. (c) Deeming, A. J.; Meah, M. N.; Bates, P. A.;Hursthouse, M. B.J. Chem. Soc., Dalton Trans. 1988, 235. (d)Deeming, A. J.; Meah, M. N.; Bates, P. A.; Hursthouse, M. B.J.Chem.Soc., Dalton Trans. 1988, 2139.

(14) Giordano, G.; Crabtree, R. H.Inorg. Synth. 1979, 19, 218.(15) Abel, E. W., Bennett, M. A.; Wilkinson, G.J.Chem. Soc. 1959, 3178.(16) Steele, D. F.; Stephenson, T. A.J. Chem. Soc., Dalton Trans. 1972,

2161.(17) Tatsuno, Y.; Yoshida, T.; Otsuka, S.Inorg. Synth. 1979, 19, 220.(18) Bonati, F.; Wilkinson, G.J. Chem. Soc. 1964, 3156.(19) Uson, R.; Oro, L. A.; Cabeza, J. A.Inorg. Synth. 1985, 23, 126.(20) Bottino, F.; Casentino, S.; Cunsolo, S.; Pappalardo, S.J. Heterocycl.

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2,6-Dimercaptopyridine Synthesis of Tetranuclear Complexes Inorganic Chemistry, Vol. 35, No. 7, 19961783

solid. The resulting suspension was concentrated, and methanol (10mL) was added in order to complete the precipitation. The solid wasfiltered off, washed with methanol, and vacuum dried. Yield: 0.112g (96%). Anal. Calcd for C42H54N2Rh4S4: C, 44.78; H, 4.83; N, 2.49.Found: C, 44.12; H, 4.79; N, 2.28.1H NMR (toluene-d8, 293 K): δ8.23 (dd, 2H,JHH ) 7.6 and 1.0 Hz), 6.73 (dd, 2H,JHH ) 8.0 and 1.0Hz), 6.29 (t, 2H,JHH ) 7.8 Hz, PyS2 ligands), 5.06 (m, 2H,dCH),4.72 (m, 10H,dCH), 4.54 (m, 2H,dCH), 3.65 (m, 2H,dCH), 2.6-2.85 (m, 12H, CH2), 2.0-2.2 (m, 8H, CH2), 1.85-2.0 (m, 8H, CH2),1.5-1.7 (m, 4H, CH2) (cod ligands). 13C NMR (CDCl3, 293 K): δ169.3 (CS), 166.8 (CS), 134.5, 125.3, 124.3 (CH, Py), 87.5 (d,JRhC )11 Hz,dCH), 85.6 (d,JRhC ) 11 Hz,dCH), 78.9 (d,JRhC ) 12 Hz,2C,dCH), 78.15 (d,JRhC ) 12 Hz, 3C,dCH), 75.9 (d,JRhC ) 12 Hz,dCH), 35.3, 33.0, 31.3 (2C), 30.4 (2C), 29.1, 28.2 (CH2 cod). MS(FAB+, CH2Cl2,m/z): 1126 (M+, 100%), 1018 (M+ - cod, 16%), 906(M+ - 2cod- 4H, 20%), 800 (M+ - 3cod- 2H, 23%), 694 (M+ -4cod, 30%). MW (CHCl3): calcd, 1126; found, 1125.Method B. [{Rh(µ-Cl)(cod)}2] (0.100 g, 0.203 mmol) was added

to a yellow solution of Li2PyS2 (0.203 mmol), prepared by addition ofn-butyllithium (0.25 mL, 1.61 M in hexane, 0.40 mmol) to a solutionof 2,6-dimercaptopyridine (0.029 g, 0.203 mmol) in tetrahydrofuran(15 mL). An orange solution was immediately formed and stirred for30 min. The solvent was removed under vacuum, and the residue waswashed with methanol (3× 10 mL) and then recrystallized fromdichloromethane (1 mL) and methanol (10 mL) to give 0.108 g of1(yield: 95%).Method C. [Rh(acac)(cod)] (0.100 g, 0.322 mmol) and 2,6-

dimercaptopyridine (0.023 g, 0.161 mmol) were stirred in dichlo-romethane (15 mL) for 30 min to give an orange solution. Concen-tration of the solution and work-up as in method B gave 0.087 g of1(yield: 97%).Method D. Solid 2,6-dimercaptopyridine (0.046 g, 0.322 mmol)

was added to a solution of [Rh(acac)(cod)] (0.100 g, 0.322 mmol) indichloromethane (15 mL) to give an orange solution. This was stirredfor 30 min, and then NEt3 (97µL, 0.70 mmol, 0.73 g mL-1) was added.The mixture was stirred for a further 1 h and then concentrated undervacuum to ca. 1 mL. Work-up as described in method B 0.086 g of1(yield: 95%).Method E. [Rh(acac)(cod)] (0.100 g, 0.322 mmol) and 2,6-

dimercaptopyridine (0.046 g, 0.322 mmol) were reacted in dichlo-romethane (15 mL) for 30 min. Solid [{Rh(µ-OMe)(cod)}2] (0.078 g,0.161 mmol) was added and the mixture stirred for 1 h. Concentrationof the solution under vacuum and work-up as described in method Bgave 0.085 g of1 (yield: 94%).[Rh4(µ-PyS2)2(nbd)4] (2) was prepared from [{Rh(µ-Cl)(nbd)}2]

(0.100 g, 0.217 mmol) and Li2PyS2 (0.217 mmol) in tetrahydrofuran(10 mL) by method B described above for1. The compoundcrystallized out in tetrahydrofuran and was isolated, after addition ofmethanol (10 mL), as a red purple solid, which was filtered, washedwith methanol, and vacuum dried. Yield: 0.101 g (88%). Anal. Calcdfor C38H38N2Rh4S4: C, 42.95; H, 3.60; N, 2.64. Found: C, 42.45; H,3.34; N, 2.59.[Rh4(µ-PyS2)2(tfbb)4] (3) was prepared from [{Rh(µ-OMe)(tfbb}2]

(0.100 g, 0.138 mmol) and 2,6-dimercaptopyridine (0.020 g, 0.138mmol) in dichloromethane (15 mL) following the method A describedabove for1. Dark red microcrystals were obtained by concentrationof the solution and slow addition of methanol (10 mL). The crystalswere isolated by filtration, washed with methanol, and vacuum dried.Yield: 0.092 g (83%). Anal. Calcd for C58H30F16N2Rh4S4: C, 43.57;H, 1.89; N, 1.75. Found: C, 43.14; H, 2.16; N, 1.86.1H NMR (CDCl3,293 K): 7.68 (d, 2H,JHH ) 9 Hz), 6.75 (t, 2H,JHH ) 9.2 Hz), 6.65 (d,2H, JHH ) 9.2 Hz) (PyS2 ligands), 5.75 (s, 6H, CH), 5.63 (m, 2H,CH), 4.41 (m, 2H,dCH), 4.34 (m, 2H,dCH), 4.27 (m, 8H,dCH),4.10 (m, 2H,dCH), 3.32 (m, 2H,dCH) (tfbb ligands). MS (FAB+,CH2Cl2, m/z): 1599 (M+ + 1H, 100%), 1372 (M+ - tfbb, 8%), 1146(M+ - 2tfbb, 5%), 1043 (M+ - 2tfbb-Rh, 6%), 920 (M+ - 3tfbb,11%), 800 (M+ - 2tfbb, - 2Rh - PyS2 + 1H, 14%), 694 (M+ -4tfbb, 22%).[Ir 4(µ-PyS2)2(cod)4] (4) was prepared from [{Ir(µ-OMe)(cod)}2]

(0.100 g, 0.151 mmol) and 2,6-dimercaptopyridine (0.021 g, 0.151mmol) in dichloromethane (10 mL). The method A described abovefor 1was followed, but exclusivelyn-hexane was added at precipitating

and washing agent. The compound was isolated by filtration as a darkred microcrystalline solid. Yield: 0.092 g (82%). Anal. Calcd forC42H54Ir4N2S4: C, 33.99; H, 3.66; N, 1.88. Found: C, 34.17; H, 3.72;N, 1.89. 1H NMR (CDCl3, 223 K): δ 7.91 (d, 2H,JHH ) 7.4 Hz),6.73 (t, 2H,JHH ) 7.6 Hz), 6.64 (d, 2H,JHH ) 7.8 Hz, PyS2 ligands),4.91 (m, 2H,dCH), 4.69 (m, 2H,dCH), 4.58 (m, 2H,dCH), 4.29(m, 2H,dCH), 3.70 (m, 2H,dCH), 3.54 (m, 2H,dCH), 2.95 (m, 2H,dCH), 2.85 (m, 2H,dCH), 2.75 (m, 2H, CH2), 2.53 (m, 2H, CH2);2.0-2.4 (set of m, 16H, CH2), 1.95 (m, 2H, CH2), 1.75 (m, 2H, CH2),1.35 (m, 8H, CH2) (cod ligands). 13C NMR (CDCl3, 293 K): δ 169.0(CS), 166.1 (CS), 135.5, 127.8, 125.9 (CH, py), 70.8, 69.5, 64.4 (br,4C), 64.0, 60.5 (dCH cod), 36.9, 34.2, 33.7 (2C), 29.3 (2C), 28.8,28.6 (CH2 cod). MS (FAB+, CH2Cl2, m/z): 1485 (M+ + 1H, 53%),1183 (M+ - Ir - cod, 100%), 1073 (M+ - Ir - 2cod- 2H, 27%),964 (M+ - Ir - 3cod- 3H, 36%), 885 (M+ - 2Ir - 2cod+ 1H,25%), 743 (M+ - 2Ir - 2cod- PyS2 + 2H, 28%).

[Pd4(µ-PyS2)2(allyl) 4] (5) was prepared by reaction of [{Pd(µ-Cl)-(allyl)}2] (0.100 g, 0.273 mmol) with Li2PyS2 (0.273 mmol) intetrahydrofuran (10 mL) according to the method B described abovefor complex1. The resulting orange solution was stirred for 1 h andthen concentrated under vacuum toca. 1 mL. Slow addition ofmethanol (5 mL) and cooling to-15 °C gave the complex as an orangemicrocrystalline solid which was filtered, washed with cold methanol,and vacuum dried. Yield: 0.090 g (76%). Anal. Calcd for C22H26N2-Pd4S4: C, 30.29; H, 3.00; N, 3.21. Found: C, 30.41; H, 3.07; N, 3.48.1H NMR (CDCl3, 293 K): δ 7.38 (m, 1H), 7.12 (m, 1H), 6.94 (m,2H), 6.69 (m, 1H), 6.62 (m, 1H, PyS2 ligands), 5.1-5.7 (4H), 3.5-4.3(set of m, 8H, syn-H), 2.3-3.5 (set of m, 8H, anti-H, allyl ligands).MS (FAB+, THF,m/z): 873 (M+ + 1H, 5%), 831 (M+ - allyl, 7%),709 (M+ - 4allyl + 1H, 4%), 391 (NBA, 100%).

[Rh4(µ-PyS2)2(CO)8] (6). Carbon monoxide was bubbled througha solution of the complex [Rh4(µ-PyS2)2(cod)4] (1) (0.221 g, 0.196mmol) in dichloromethane (10 mL) for 20 min. The color of thesolution turned dark red, and then ethanol (15 mL) was added. Thesolvents were distilled under an atmosphere of carbon monoxide up to4 mL to remove the displaced cyclooctadiene. Cooling of the solutionat room temperature under a carbon monoxide atmosphere gave thecomplex as violet-black dichroic crystals, which were washed with coldethanol and then dried under vacuum. Yield: 0.121 g (67%). Anal.Calcd for C18H6N2O8Rh4S4: C, 23.55; H, 0.66; N, 3.05. Found: C,23.78; H, 0.85; N, 2.99.1H NMR (CDCl3, 293 K): δ 7.37 (dd, 2H,Hm), 7.00 (dd, 2H, Hm′), 6.84 (dd, 2H, Hp) (J ) 1.0 Hz,JHm-Hp ) 7.5Hz, JHm′-Hp ) 8.1 Hz) (PyS2 ligands). 13C NMR (CDCl3, 223 K): δ184.3 (d,JRhC ) 64 Hz, CO), 182.2 (d,JRhC ) 73 Hz, CO), 182.0 (d,JRhC ) 72 Hz, CO), 181.6 (d,JRhC ) 64 Hz, CO), 168.6 (CS), 159.2(CS), 136.9, 127.1, 123.0 (CH, Py). MS (FAB+, toluene,m/z): 918(M+, 16%), 890 (M+ - CO, 24%), 862 (M+ - 2CO, 40%), 834 (M+

- 3CO, 60%), 806 (M+ - 4CO, 15%), 778 (M+ - 5CO, 18%), 750(M+ - 6CO, 28%), 722 (M+ - 7CO, 15%), 694 (M+ - 8CO, 30%),488 (M+ - 2Rh- 8CO, 73%), 443 (Rh(CO)2(PyS2H)2, 100%). IR(hexane, cm-1): ν (CO), 2090 (m), 2072 (s), 2062 (s), 2024 (s), 2012(m).

[Rh4(µ-PyS2)2(CO)4)(PPh3)4] (7) was prepared from [{Rh(µ-Cl)-(CO)(PPh3)}2] (0.250 g, 0.291 mmol), 2,6-dimercaptopyridine (0.041g, 0.291 mmol), andn-BuLi (0.36 mL, 1.6 M inn-hexane, 0.583 mmol)in tetrahydrofuran (15 mL) following the method B described abovefor complex1. The dark red solution was concentrated under vacuumto ca. 1 mL, diethyl ether (5 mL) and methanol (10 mL) were slowlyadded, and the mixture was again concentrated. A red-violet solidcrystallized out, which was washed repeatedly with cold methanol andthen vacuum dried. Yield: 0.200 g (74%). Complex7 can bealternatively synthesized by reaction of [Rh(acac)(CO)(PPh3)] (0.075g, 0.152 mmol) with Py(SH)2 (0.022 g, 0.152 mmol) and NEt3 (42µL,0.30 mmol, 0.73 g mL-1) in dichloromethane (5 mL). Work-up asabove gave7 in 82% yield (0.058 g). Anal. Calcd for C86H66N2O4P4-Rh4S4: C, 55.68; H, 3.59; N, 1.51. Found: C, 55.37; H, 3.47; N, 1.63.31P NMR (CDCl3, 218 K): δ 43.3 (d,JRhP) 160 Hz), 41.5 (d,JRhP)167 Hz), 41.2 (d,JRhP ) 159 Hz), 40.1 (d,JRhP ) 166 Hz), 40.0 (d,JRhP) 166 Hz), 39.6 (d,JRhP) 167 Hz), 38.2 (d,JRhP) 150 Hz), 37.5(d, JRhP ) 167 Hz, mixture of isomers). MS (FAB+, acetone,m/z):

1784 Inorganic Chemistry, Vol. 35, No. 7, 1996 Perez-Torrente et al.

1855 (M+, 80%), 1480 (M+ - 4CO- PPh3, 31%), 1218 (M+ - 4CO- 2PPh3, 35%), 391 (NBA, 100%). IR (CH2Cl2, cm-1): ν(CO), 1975(br, s).[Rh2(µ-PyS2H)2(cod)2] (8). To a solution of [Rh(acac)(cod)] (0.150

g, 0.428 mmol) in dichloromethane (10 mL), solid 2,6-dimercaptopy-ridine (0.069 g, 0.482 mmol) was added to give a dark orange solutionwhich was stirred for 30 min. Concentration of the solution resultedin partial precipitation of a solid which was completed by addition ofa mixture of methanol-diethyl ether (1:1). The solid was isolated byfiltration and washed with methanol. Complex8 is obtained as anorange-brown solid by recrystallization from dichloromethane/methanol.Yield: 0.132 g (78%). Complex8 can be alternatively synthesizedby reaction of Py(SH)2 (0.059 g, 0.410 mmol) with the solvated species[Rh(cod)(Me2CO)x][BF4] (0.410 mmol) prepared “in situ” by reactionof [{Rh(µ-Cl)(cod)}2] (0.101 g, 0.205 mmol) with AgBF4 (0.080 g,0.410 mmol) in acetone. Work-up as described above gave8 in 94%yield (0.136 g). Anal. Calcd for C26H32N2Rh4S4: C, 44.19; H, 4.56;N, 3.96. Found: C, 43.66; H, 4.38; N, 4.00. MS (FAB+, CH2Cl2,m/z): 916 (M+ + Rh+ cod, 41%), 706 (M+, 37%), 598 (M+ - cod,29%), 564 (M+ - PyS2, 44%), 391 (NBA, 100%), 354 (M+ - Rh-cod- PyS2, 33%). MW (CHCl3): calcd, 706; found, 873.Synthesis of Heterotetranuclear Complexes.In a typical procedure

[M(acac)(cod)] (0.300 mmol) (M) Rh, Ir) and 2,6-dimercaptopyridine(0.003 mmol) were reacted in dichloromethane (10 mL) for 15 min.Solid [{M′(µ-OMe)(cod)}2] (M ′ ) Rh, Ir; M * M′) was then added togive a purple-red solution. Concentration of the solution toca. 1 mLand slow addition of methanol gave purple microcrystalline solids whichwere filtered, washed with methanol, and then vacuum dried. Char-acterization of the complexes was achieved by1H NMR and MS spectra(see text).Crystal Structure Determination of [Rh 4(µ-PyS2)2(cod)4] (1).

Suitable crystals for the X-ray diffraction study were obtained by slowdiffusion of methanol into a concentrated solution of1 in a dichlo-romethane/diethyl ether mixture. The selected crystal was a redtransparent irregular block of approximate dimensions 0.086× 0.137× 0.315 mm. Cell parameters were determined from 64 accuratelycentered reflections in the range 20e 2θ e 47°. Intensity data wererecorded usingω/2θ scans (3e 2θ e 47°) on a Siemens-Stoe AED-2diffractometer equipped with a highly oriented graphite crystal mono-chromator. Crystal data and details associated with structure refinementare summarized in Table 1. Three standards reflections were monitoredduring data collection every 55 min of measuring time; crystal decay(e1%) was corrected according the intensities of the standard reflec-tions. Data were corrected for Lorentz and polarization effects, and asemiempirical correction, based on azimuthalψ-scans from sevenreflections, was also applied.22

The structure was solved by Patterson and Fourier methods. Allnon-hydrogen atoms were isotropically and subsequently anisotropicallyrefined by full-matrix least-squares. All of the H-atoms were locatedand included in the final steps of refinement as free isotropic atoms.Residuals in the∆F map were 0.35 e/Å3. Scattering factors, corrected

for anomalous dispersion, were taken from ref 23. All calculationswere performed on aµ-VAX 3400 computer with the SHELXTL-PLUSpackage.24 Final atomic coordinates and equivalent temperature factorsare given in Table 2.

Results and Discussion

Tautomerism of the Ligand. In the original preparations20,21

of the ligand 2,6-dimercaptopyridine [Py(SH)2] the 1H NMRspectrum was not described, probably because it is temperatureand solvent dependent. At low temperature in toluene there isa single compound, the 3-mercapto-2(1H)-pyridinethione tau-tomer (A), in agreement with the1H NMR spectrum (seeExperimental Section), which shows five well-defined reso-nances: three for the aromatic protons and two for the NH andSH protons. The two latter signals broaden as the temperatureincreases and become a broad band at room temperature whichshould be due to the acidic proton exchange. Further heatingleads to broadening of themeta proton resonances, whicheventually coalesce and became equivalent at high temperature.A reasonable explanation for the chemical equivalence of themetaprotons is a shift to the right of the tautomeric equilibriumin Scheme 1 on raising the temperature. However, the1H and13C{1H} spectra in CDCl3 at room temperature are consistentwith structure B (see Experimental Section). Obviously, the

(22) North, A. C. T.; Phillips, D. C.; Mathews, F. S.Acta Crystallogr.1968, A24, 351.

(23) International Tables for X-Ray Crystallography; Kynoch Press:Birmingham, England, 1974; Vol. 4.

(24) Sheldrick, G. M. SHELXTL PLUS Program for Crystal StructureSolution and Refinement; Siemens Analytical X-ray Instruments,Madison, WI, 1990.

Table 1. Summary of Crystal data for [Rh4(µ-pyS2)2(cod)4] (1)

formula C42H54N2Rh4S4 Dcalcd, g cm-3 1.881mol wt 1126.8 λ (Mo KR) radiation, Å 0.710 73cryst size, mm 0.086× 0.137× 0.315 temp, K 298cryst syst monoclinic µ, mm-1 1.874space group C2/c (No. 15) transmn facts 0.636, 0.776a, Å 10.252(1) 2θ range, deg 3-47b, Å 17.023(2) no. of data collctd 6381 (+h, (k, (l)c, Å 23.114(3) no. of unique data 2925 (Rint ) 0.025)â, deg 99.50(1) unique obsd data 2393 [(Fo) g 4σ(Fo)]V, Å3 3978(1) R, Rwa 0.0228, 0.0240Z 4

a R ) ∑[|Fo| - |Fc|]/∑|Fo|; Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo|2]1/2; w-1 ) σ2(Fo) + 0.0001Fo2.

Table 2. Atomic Coordinates (×104; ×105 for Rh and S Atoms)and Equivalent Isotropic Displacement Coefficientsa (Å2, ×104) forthe Complex [Rh4(µ-pyS2)2(cod)4] (1)

atom x/a y/b z/c Ueq

Rh(1) 22586(3) 95475(2) 13079(1) 293(1)Rh(2) -2713(3) 87364(2) 16403(1) 266(1)S(1) 3713(11) 103524(7) 10510(5) 370(4)S(2) 18082(10) 90630(6) 22256(4) 286(3)N -931(3) 9820(2) 1891(1) 254(11)C(1) -639(4) 10458(2) 1581(2) 290(13)C(2) -1149(5) 11194(3) 1694(2) 401(16)C(3) -1882(5) 11275(3) 2130(2) 443(17)C(4) -2111(5) 10636(3) 2464(2) 369(16)C(5) -1597(4) 9916(2) 2347(2) 259(13)C(6) 2374(5) 9514(3) 383(2) 415(17)C(7) 3088(5) 10157(3) 628(2) 408(17)C(8) 4589(5) 10196(3) 765(2) 499(20)C(9) 5120(5) 9970(3) 1385(2) 500(20)C(10) 4302(5) 9356(3) 1618(2) 444(18)C(11) 3766(5) 8684(3) 1329(2) 401(17)C(12) 3994(7) 8422(4) 728(3) 545(22)C(13) 2952(6) 8745(4) 240(2) 534(20)C(14) -2244(5) 8358(3) 1318(2) 408(17)C(15) -1743(5) 8741(3) 878(2) 406(16)C(16) -1221(7) 8342(3) 379(2) 531(20)C(17) -307(6) 7664(3) 572(2) 524(21)C(18) 439(5) 7765(3) 1186(2) 396(17)C(19) 21(5) 7502(3) 1681(2) 403(17)C(20) -1276(6) 7077(3) 1688(3) 511(20)C(21) -2445(6) 7485(3) 1352(3) 541(21)

a Equivalent isotropicU defined as one-third of the trace of theorthogonalizedUij tensor.


tautomeric equilibrium is influenced by the solvent, and polarsolvents favor the shift to the more polar tautomer B.Homotetranuclear Diolefin Complexes.Protonation of the

methoxo groups in the dinuclear complexes [M(µ-OMe)-(diolefin)]2 (M ) Rh, Ir) with binucleating ligands containingacidic protons is a convenient synthetic approach to dinuclearcomplexes, even if the pKMeOH is smaller than pKligand.25 Inour case, the reaction of Py(SH)2 with [{Rh(µ-OMe)(cod)}2](cod ) 1,5-cyclooctadiene) in a molar ratio 1:1 gives thetetranuclear complex [Rh4(µ-PyS2)2(cod)4] (1), which is isolatedas orange microcrystals in high yield (method A). Alternatively,the ligand Py(SH)2 can be doubly deprotonated in situ byn-butyllithium in tetrahydrofuran to give a pale yellow solution.Further reaction of this solution with 1 equiv of [{Rh(µ-Cl)-(cod)}2] affords a mixture of1 and lithium chloride, which iseasily separated due to the insolubility of1 in methanol (methodB). This alternative synthetic route gives1 in similar yield tothat of route A but is less convenient since it requires the useof n-butyllithium.Complex1 is soluble in dichloromethane, tetrahydrofuran,

and toluene, sparingly soluble in diethyl ether and hexanes, andinsoluble in MeOH. Complex1was formulated as tetranuclearfrom mass spectroscopy, which shows the molecular ion at 1126(M+, 100%) with sequential loss of four 1,5-cyclooctadieneligands. The IR spectrum shows three strong characteristicabsorptions at 1560, 1540, and 1410 cm-1 due to the thioamideν(CdN) + ν(SdCsN) modes26 of 2,6-dimercaptopyridine, andthe absence of bothν(SH) andν(NH) absorptions in the IRconfirms the presence of doubly deprotonated ligands in thecomplex.The related diolefin rhodium complexes, [Rh4(µ-PyS2)2(nbd)4]

(2) and [Rh4(µ-PyS2)2(tfbb)4] (3) (nbd ) norborna-2,5-diene,tfbb) tetrafluorobenzo[5,6]bicyclo[2.2.2]octa-2,5,7-triene), havebeen also prepared following the methods described for1.Complex2 is isolated as a red-purple solid insoluble in the usualorganic solvents, while complex3 is a dark red solid with greatersolubility which enabled its characterization by mass and NMRspectroscopies. Both show the characteristic strong thioamideabsorptions in their IR spectra at 1560, 1540, and 1410 cm-1,probably associated with the coordination mode of the PyS2

2-

ligand in the tetranuclear framework.Other tetranuclear complexes of different metals can be

prepared by the above described routes. So, the iridium(I) andpalladium(II) complexes [Ir4(µ-PyS2)2(cod)4] (4) and [Pd4(µ-PyS2)2(allyl)4] (5) result from the reaction of the correspondingchloro complexes [{M(µ-Cl)L2}2] (M ) Ir, L2 ) cod; M) Pd,L2 ) allyl) with a solution of Li2PyS2 prepared in situ. Theyare isolated in high yield as black-red and orange microcrys-talline solids, respectively. Complex4 is also accessible bydirect protonation of the methoxo ligands in [{Ir(µ-OMe)-(cod)}2]. The tetranuclear formulation of complexes4 and5is confirmed by their mass spectra, which show the molecularions at the expected values ofm/z.In order to establish the coordination of the bridging ligands

and to obtain accurate geometrical parameters to elucidate the

fluxional behavior of the tetranuclear complexes, a single crystalX-ray analysis of complex1 was carried out. Figure 1 showsa representation of the molecule together with the atom labelingscheme used. Table 3 displays selected bond distances andangles.The crystal structure of1 consists of tetranuclear complexes

packed with no close intermolecular contacts. The moleculeexhibits a crystallographically imposedC2 symmetry with onlyone independent “Rh2(µ-PyS2)(cod)2” moiety in the asymmetricunit. The two related 2,6-pyridinedithiolate groups bridge allof the four metal centers acting as tridentate ligands; they arebonded through the pyridinic nitrogen and through both sulfuratoms with one of them coordinated in aµ2 fashion to twodifferent metal centers. Each rhodium has a slightly distortedsquare-planar environment with a chelating cyclooctadienemolecule interacting through the two olefinic bonds. Theshortest intermetallic separation, Rh(1)‚‚‚Rh(2), is 3.1435(5) Åand excludes any but the weakest interaction between the metals;the Rh(2)‚‚‚Rh(2′) distance across theC2 axis is significantlylonger, 3.9210(6) Å.

(25) (a) Ciriano, M. A.; Pe´rez-Torrente, J. J.; Oro, L. A.J. Organomet.Chem. 1993, 445, 267. (b) Oro, L. A.; Fernandez, M. J.; Modrego,J.; Lopez, J. M.J. Organomet. Chem. 1985, 287, 409.

(26) Raper, E. S.Coord. Chem. ReV. 1985, 61, 115.

Scheme 1

Figure 1. View of the molecular structure of [Rh4(µ-PyS2)2(cod)4] (1)showing the atom labeling scheme. Hydrogen atoms have been labeledwith the number of the carbon to which they are bonded.

Table 3. Selected Bond Lengths (Å) and Anglesa (deg) for[Rh4(µ-pyS2)2(cod)4]

Rh(1)‚‚‚Rh(2) 3.1435(5 Rh(2)‚‚‚Rh(2′) 3.9210(6)Rh(1)-S(1) 2.365(1) Rh(2)-N 2.080(3)Rh(1)-S(2′) 2.391(1) Rh(2)-S(2) 2.394(1)Rh(1)-C(6) 2.161(4) Rh(2)-C(14) 2.136(5)Rh(1)-C(7) 2.171(5) Rh(2)-C(15) 2.123(4)Rh(1)-C(10) 2.125(5) Rh(2)-C(18) 2.149(5)Rh(1)-C(11) 2.128(5) Rh(2)-C(19) 2.122(4)C(6)-C(7) 1.383(7) C(14)-C(15) 1.378(7)C(10)-C(11) 1.392(7) C(18)-C(19) 1.361(7)S(1)-C(1) 1.739(4) S(2)-C(5) 1.789(4)N-C(1) 1.362(5) N-C(5) 1.356(5)C(1)-C(2) 1.398(6) C(4)-C(5) 1.378(6)C(2)-C(3) 1.361(7) C(3)-C(4) 1.375(7)

S(1)-Rh(1)-S(2′) 98.79(4) N-Rh(2)-S(2) 86.4(1)S(1)-Rh(1)-M(1) 86.6(1) N-Rh(2)-M(3) 91.2(1)S(1)-Rh(1)-M(2) 170.0(1) N-Rh(2)-M(4) 174.4(1)S(2′)-Rh(1)-M(1) 173.3(1) S(2)-Rh(2)-M(3) 174.5(1)S(2′)-Rh(1)-M(2) 88.8(1) S(2)-Rh(2)-M(4) 96.4(1)M(1)-Rh(1)-M(2) 86.4(1) M(3)-Rh(2)-M(4) 86.4(1)Rh(1)-S(1)-C(1) 116.1(1) Rh(1)-S(2′)-Rh(2) 82.13(3)S(1)-C(1)-N 119.8(3) Rh(1)-S(2′)-C(5′) 105.6(1)S(1)-C(1)-C(2) 120.6(3) Rh(2)-S(2′)-C(5′) 108.8(1)N-C(1)-C(2) 119.6(4) Rh(2)-N-C(1) 116.7(2)S(2)-C(5)-N 117.0(3) Rh(2)-N-C(5) 123.5(2)S(2)-C(5)-C(4) 121.9(3) C(1)-N-C(5) 119.7(3)N-C(5)-C(4) 121.1(3)

a Primed atoms are related to the unprimed ones by the symmetrytransformation-x, y, 1/2 - z. M(1), M(2), M(3) and M(4) are themidpoints of the olefinic bonds C(6)-C(7), C(10)-C(11), C(14)-C(15), and C(18)-C(19), respectively.


The 2,6-pyridinedithiolate bridging ligand is roughly planar.The sulfur atoms display deviations from the pyridinic ring of0.144(1) and 0.104(1) Å for S(1) and S(2), respectively. Thedihedral angle between the two symmetry related dithiolateligand planes is 26.7(1)°. If compared with the related2-mercaptopyridine free base,27 the most remarkable feature isthe elongation of the C-S bond distances from 1.692(2) Å, inthe free related ligand, to 1.739(4) (C(1)-S(1)) and 1.789(4)Å (C(5)-S(2)). These changes clearly evidence that one ofthe C-S bonds maintain a clear thione character, as aconsequence of the electronic donation upon coordination tothe metal. As expected, the C(5)-S(2) distance is longer thanC(1)-S(1), corresponding with theµ2 character of the S(2) atom.However, both C-S distances are significantly shorter thantypical C-S single bonds, for instance, 1.829(26) Å in al-kanethiolates,28 showing the partial aromaticity of the bridgingligands.The Rh-S bond distances (2.365(1) Å for Rh(1)-S(1), the

slightly longer distances for the bridging S(2) atom (2.391(1)and 2.394(1) Å), and the Rh-N length (2.080(3) Å) comparewell with complexes containing the closely related N-S donorligands as benzothiazole-2-thiolate or pyridine-2-thiolate (meanvalues 2.38(8) Å for Rh-S and 2.13(7) Å for Rh-N).29 Thesquare-planar metal environments are slightly distorted andexhibit deviations always under 0.215(4) Å. Both independentmetal coordination planes are almost perpendicular making adihedral angle of 97.0(1)°.Each cyclooctadiene ring has a boat conformation with a

mean carbon-carbon double bond length of 1.379(6) Å.Interestingly, within the accuracy obtained, the asymmetryobserved in the other side of the metal coordination spheresdoes not affect significatively the coordination of the diolefinicmolecules; only the Rh(1)-C bond distancestrans to the S(2′)are slightly longer than the rest.NMR Studies on the Diolefin Complexes. The diolefin

complexes1, 3, and4 behave in a similar way, judging fromtheir 1H and 13C NMR spectra. They show equivalent andapparently static bridging ligands, while there is a fluxionalmovement associated with the diolefin groups. This nonrigidityin a polynuclear complex was intriguing and was investigatedfurther.Figure 2 shows the variable temperature1H NMR spectra of

the diolefinic region for complex4 as a representative example.Eight well-defined olefinic proton resonances are observed forthe complexes at low temperature. In addition, their13C{1H}NMR spectra show also eight resonances for the olefinic carbonsin the slow-exchange region. Moreover, the resonances for theC-S carbons of the pyridine ring appear in the13C{1H} NMRspectra as two different singlets as a consequence of thecoordination mode of the bridging ligand. These data are inaccordance with the structure found in the solid state for1,where there are two types of equivalent diolefin ligands, theinner and the external. In addition, molecular weight determi-nations for complexes1, 3, and4 confirm that the species insolution are tetranuclear. Hence, all the olefinic complexes areconcluded to have the structure found for complex1 in solution.TheC2 symmetry of the tetranuclear structure let us simplifythe discussion by treating the complexes as dinuclear havingthe A-frame structure of half of the molecule,i.e. two differentbridges and hence lacking any element of symmetry.

The H,H-COSY spectra at low temperature allow theidentification of the four pairs of resonances related by coupling,i.e., those of the protons bonded to a given CdC bond. Namely,they are those atδ 4.95 and 4.60, 4.70 and 3.75, 4.30 and 2.85,and 3.60 and 2.95 for complex4 (Figure 3a). Further informa-tion for assigning the resonances from each diolefin wasobtained from the variable temperature spectra, since they showthat one type of the diolefin ligands is rigid whereas the other

(27) Ohms, U.; Guth, H.; Kutoglu, A.; Scheringer, C.Acta Crystallogr.1982, B38, 831.

(28) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.G.; Taylor, R.J. Chem. Soc., Dalton Trans. 1991, S1.

(29) Ciriano, M. A.; Pe´rez-Torrente, J. J.; Lahoz, F. J.; Oro, L. A.J.Organomet. Chem. 1993, 455, 225.

Figure 2. Variable temperature spectrum of [Ir4(µ-PyS2)2(cod)4] (4)at (a) 223, (b) 293, and (c) 328 K in CDCl3. The water and pentanepeaks are denoted by asterisks.

Figure 3. (a) H,H-COSY spectrum of the diolefin region of [Ir4(µ-PyS2)2(cod)4] (4) at the slow-exchange region and (b) map of the“mobile” diolefin.


undergoes a fluxional motion. Figure 2b corresponds to thespectrum of4 at the temperature of coalescence, showing thatthe resonances atδ 4.70 (H(2)) and 3.75 (H(5)), 4.30 (H(4))and 2.85 (H(8)) are due to the rigid diolefin. Furthermore, theH,H-COSY spectrum at this temperature (Figure 4a) allows theconstruction of the map of this diolefin as shown in Figure 4b.Therefore, H(2) is coupled with the olefinic H(5) and with themethylene exo and endo protons H(11) and H(19); H(19) isalso coupled to H(18) and H(23), which, in turn, are coupledto the olefinic H(8). In a similar way, the relationships betweenthe protons of the diolefin to complete the cycle can be easilylocated in this COSY spectrum. The maps of the rigid and“mobile” diolefins can be calculated in a similar way from thelow-temperature COSY spectrum (Figure 3a). Therein aredepicted the relationships between the protons of a half of thecycle of the mobile diolefin by example. Therefore, H(6) iscoupled with the olefinic H(7) and with the methylene exo andendo protons H(12) and H(20); H(20) is also coupled to H(17)and H(24), which, in turn, are coupled to the olefinic H(3). Themap of this diolefin is in Figure 3b.Once the two sets of resonances due to the two distinct

diolefin ligands have been identified, two problems arise: firstly,the decision of which diolefin is which and secondly theassignment of the olefin signals within each. To address thesepoints, another technique independent of the chemical shiftshould be desirable such as the nuclear Overhauser effect (NOE)previously used for structural purposes. Nevertheless, theolefinic protons are far away from those of the bridging ligands,and NOE effects between them should not be observable. Onthe other hand, a careful observation of the structure of1 in thesolid state allows the detection of a close proximity (2.57 Å)

between two olefinic protons, one from each diolefin ligand,namely, those bonded to C(11) and C(18) in Figure 1.Identification of these resonances by NOE experiments wouldallow the full assignment of the diolefinic protons. Unfortu-nately NOESY experiments on complex4 are not conclusivesince chemical exchange still occurs at 225 K and the assign-ments need to be done on the basis of the chemical intuition.Closer inspection of the crystal structure reveals a differencebetween both types of ligands. The inner diolefin istrans tothe nitrogen and a bridging sulfur, while the outer olefin istransto a bridging and to a terminal sulfur. In our experience withcomplexes containing bridging N-C-S ligands, the presenceof free lonely pairs on the sulfur donor atom give rise tofluxionality12e due to the well-known inversion at the sulfur,30

while static NMR are shown by either complexes with a metal-metal bond12a or trinuclear complexes where all the donorelectron pairs of the bridging ligand are involved in coor-dination.13a,b On this basis, the rigid diolefin should be the innerone, where the lone pairs of thetrans sulfur are engaged.To assign the protons of the rigid diolefin, it should be noted

that those resonances at lower field (H(2) and H(5)) correspondto the olefinic bondtrans to the nitrogen, as pointed out byMann31 for open-book dinuclear 2-pyridonate iridium com-plexes. Therefore, H(4) and H(8) aretransto the bridging sulfuratom. Furthermore, for the mobile diolefin H(1) and H(3)should betrans to the terminal sulfur atom and, hence, H(6)and H(7)trans to the bridging sulfur. No further assignmentcan be made on the basis of the data available, but it isremarkable that the chemical shift of the olefin resonances isinfluenced in the same amount both by their relative positionin the molecule, being inside or outside of the A-frame, and bythe atomtrans to them.Inspection of the1H and13C{1H} NMR spectra of4 at high

temperature reveals that two pairs of the olefinic protons andcarbons of the outer diolefin became equivalent in the fastexchange region. Moreover, these signals result from twodifferent CdC bonds, and the NOESY spectrum of4 confirmsthat the interchanging protons are H(1) with H(6) and H(3) withH(7), i.e., those related by a rotation of the diolefin ligand aroundan axis passing through the metal, coincident with the bisectrixof the S-M-S angle. As the13C{H} NMR spectrum shows,the pyridine CS resonances are unaffected and well-differenti-ated. No change in the coordination of the bridging ligandsoccurs, and hence, the motion of the diolefin can be describedas a rotation assisted by the inversion at the sulfur. The energyof activation for this motion can be estimated in 12 kcal/molusing the DNMR6 program,32 a value expected for the rotationof olefin ligands.33

On the other hand, the tetranuclear palladium complex5existsin solution as a mixture of isomers, as deduced from its1HNMR spectrum, and was not investigated further.Homotetranuclear Rhodium Carbonyl Complexes. Com-

plex [Rh4(µ-PyS2)2(CO)8] (6) is obtained by carbonylation of1in dichloromethane at atmospheric pressure. The carbonylationprocess is partially reversible since when freshly made solutionsof complex6 are concentrated under vacuum in the presenceof cyclooctadiene, the isolated black residue shows the molecularion for the species [Rh4(µ-PyS2)2(CO)6(cod)] (m/z ) 971).

(30) Abel, E. W.; Orrell, K. G.; Bhargava, S. K.Prog. Inorg. Chem. 1984,32, 1.

(31) Rodman, G. S.; Mann, K. R.Inorg. Chem. 1988, 27, 338.(32) Brown, J. H.; Bushweller, C. H.DNMR6: Calculation of NMR Spectra

(I ) 1/2) Subject to the Effects of Chemical Exchange; QCPE ProgramNo. 633; Indiana University: Bloomington, IN.

(33) Vierkotter, S. A.; Barnes, C. E.; Garner, G. L.; Butler, L. G.J. Am.Chem. Soc. 1994, 166, 7445.

Figure 4. (a) H,H-COSY spectrum of the diolefin region of [Ir4(µ-PyS2)2(cod)4 (4) nearly at the coalescence temperature and (b) map ofthe static diolefin ligand.


Isolation of pure complex6 requires the removal of the replacedcyclooctadiene by distillation with ethanol. Alternative routesto 6 give poorer results. For example, the reaction of Li2PyS2and [{Rh(µ-Cl)(CO)2}2] in tetrahydrofuran or toluene givespyrophoric black-green solids.Complex6 is isolated as dark-violet dichroic crystals which

turn into a green solid when pulverized. Solutions of6 in anysolvent are intensively colored and exhibit red-green dichroism.The tetranuclear framework of1 is maintained on carbonylation.So, the mass spectrum shows the molecular ion atm/z ) 918with sequential loss of eight carbonyl ligands. In addition, the13C{1H} NMR spectrum at low temperature shows equivalentbridging ligands and four doublets for the carbonyl groups dueto coupling to the rhodium-103 active nuclei in accordance witha structure similar to that found for1 but with carbonyl groupsas ancillary ligands. The presence of bridging and terminalS-donor atoms is determined by two distinctive resonances forthe C(S) atoms of the 2,6-pyridinedithiolate ligands, as observedin the13C{1H} spectra of complex1. On warming, two of thecarbonyl doublets collapse and a new doublet emerges at themidway point so that the two carbonyl ligands becameequivalent. This effect should be attributed again to theinversion at the sulfur atoms coordinated to the external rhodiumdicarbonyl groups.Freshly made solutions of complex6, prepared by reaction

of 1 with carbon monoxide in dichloromethane, react with 4equiv of triphenylphosphine to give solutions that contain [Rh4-(µ-PyS2)2(CO)4(PPh3)4] (7) and other unidentified Rh(I) species.Complex7 is obtained pure in high yield as a red-purple solidby reaction oftrans-[Rh(µ-Cl)(CO)(PPh3)]2 with Li2PyS2 intetrahydrofuran. The IR spectrum of7 in dichloromethaneshows a broad absorption at 1975 cm-1, suggesting that eachrhodium atom contains a carbonyl and a triphenylphosphineligand; otherwise the presence of the “cis-Rh(CO)2” units shouldbe detected.34 The31P{1H} NMR spectrum at room temperatureconsists of a broad doublet on a background of broad resonancesranging from 43 to 35 ppm. When the spectrum is recorded at223 K (Figure 5a), doublets from a mixture of different speciesare observed. Bearing in mind the framework of complex1, alarge number of isomers are possible from the relative disposi-tion of the CO and PPh3 ligands on each rhodium center.Nevertheless, replacement of carbonyl ligands by triphenylphos-phine in related tetracarbonyl dinuclear rhodium complexeshaving N and S donor atoms occurstrans to the sulfur; forexample in [Rh2(µ-bzta)2(CO)3(PPh3)] (bzta: benzothiazole-2-thiolate) and [{Rh(µ-bzta)(CO)(PPh3)}2].29 In a similar way,replacement of the carbonyl ligands in the assymmetric bridgedbinuclear complex [Rh2(µ-pz)(µ-StBu)(CO)4] occurs exclusivelytrans to the thiolate ligand.35 Taking this into consideration,the triphenylphosphine ligands should betrans to the sulfuratoms in the two inner rhodium atoms coordinated both to Nand S. Therefore, the observed mixture should be made fromisomers having different relative dispositions of both carbonyland triphenylphosphine ligands on the external rhodium atoms.Figure 5b shows the three possible isomers resulting from thisconsideration. Interpretation of the31P{1H} NMR spectrum isnow straightforward: the low-field doublet resonances at 43.3and 41.5 ppm are assigned to aC2 isomer (b), where two setsof equivalent phosphine ligands are expected; the high-fielddoublets at 38.2 and 37.5 ppm are attributed to the otherC2

isomer (*), and finally, the resonances at 41.2, 40.1, 40.0, and39.6 ppm correspond to theC1 isomer. The observed couplingconstants (1JRhP) around 160 Hz are in accordance withtriphenylphosphine ligands trans to S-donor atoms.36

Approach to the Synthesis of Heterotetranuclear Com-plexes. Protonation of acetylacetonate complexes [M(acac)-(diolefin)] or methoxide bridged complexes [{M(µ-OMe)-(diolefin)}2] with binucleating ligands containing acidic protonsgive similar results for the synthesis of binuclear rhodium(I) oriridium(I) complexes,37 although sometimes unexpected com-pounds have been prepared following the “acac route”.38 Theprotonation of [Rh(acac)(cod)] with Py(SH)2 can be carried outstepwise. Using a molar ratio of 1:1, the dinuclear complex[Rh2(µ-PyS2H)2(cod)2] (8) is isolated, while in a molar ratio of2:1 a tetranuclear aggregate [Rh4(µ-PyS2)2(cod)4] (1) is obtained(method C, Experimental Section). A small amount of1 is alsoformed in the synthesis of8, which is easily removed byrecrystallization. Complex8 can be alternatively prepared byreaction of 2,6-dimercaptopyridine with the solvated species

(34) (a) Bruce, M. I.; Goodall, B. L.; Iqbal, M. Z., Stone, F. G. A.J.Chem.Soc.,Chem. Commun. 1971, 661. (b) Khan, Md. N. I.; Fackler, J. P.,Jr.; King, C.; Wang, S.Inorg. Chem. 1988, 27, 1672.

(35) Claver, C.; Kalck, P.; Ridmy, M.; Thorez, A.; Oro, L. A.; Pinillos,M. T.; Apreda, M. C.; Cano, F. H.; Foces-Foces, C.J. Chem. Soc.,Dalton Trans. 1988, 1523.

(36) Osakada, K.; Matsumoto, K.; Yamamoto, A.Organometallics1985,4, 857.

(37) (a) Oro, L. A.; Pinillos, M. T.; Tejel, C.; Apreda, M. C.; Foces-Foces,C.; Cano, F. H.J. Chem. Soc.,Dalton Trans. 1988, 1927. (b) Oro, L.A.; Pinillos, M. T.; Tejel, C.; Foces-Foces, C.; Cano, F. H.J. Chem.Soc., Dalton Trans. 1986, 1087.

(38) (a) Calvo, M. A.; Manotti-Lanfredi, A. M.; Oro, L. A.; Pinillos, M.T.; Tejel, C.; Tiripicchio, A.; Ugozzoli, F.Inorg. Chem. 1993, 32,1147. (b) Oro, L. A.; Ciriano, M. A.; Villarroya, B. E.; Tiripicchio,A.; Lahoz, F. J.J. Chem. Soc., Chem. Commun. 1984, 521.

Figure 5. 31P{1H} NMR spectrum of [Rh4(µ-PyS2)2(CO)4(PPh3)4] (7)at 223 K (a) with the assignation to the isomers (b).


[Rh(cod)(Me2CO)x][BF4] in acetone. HBF4 is thus producedin the reaction upon binding of the Py(SH)2 ligands to rhodium(Scheme 2).Characterization of8 as dinuclear is based upon analytical

results. Its mass spectrum shows the molecular ion at theexpectedm/z ) 706 (37%), but also the trinuclear aggregate[Rh3(PyS2H)2(cod)3]+ (m/z) 916, 12%) is detected. Complex8 is a nonconductor in dichloromethane, and its molecularweight in chloroform is invariantly higher than the expected,suggesting an associative process in solution. In fact, the1HNMR spectrum shows a mixture of two species containingPyS2H- ligands, showing resonances at 13.00 and 12.37 ppmassignable to NH and a signal at 5.56 ppm for the SH protons.The broad resonances at 12.37, 6.91, 4.30, 2.46, and 1.95 ppmcorrespond to the main component which is fluxional. Theminor species has inequivalent PyS2H- ligands which displaysix resonances at 7.45 (d), 7.30 (d), 6.90 (m), 6.65 (m), and6.55 ppm; the cyclooctadiene resonances are shown as multipletswidespread between 5.1 and 1.5 ppm. In agreement with thespectroscopic information the structural proposals are adventur-ous in view of the possible coordination modes of the PyS2H-

ligands. In spite of the fact that complex [Rh2(µ-PyS2H)2(cod)2](8) exists as a mixture in solution, it behaves as the precursorof the tetranuclear aggregate1 (Scheme 2) by deprotonationwith 2 equiv of NEt3 (method D). Interestingly, deprotonationof the bridging ligands can be accomplished by methoxo bridgeddinuclear complexes. Therefore, a solution of8 reacts with 1equiv of [{Rh(µ-OMe)(cod)}2] to give1 in high yield (methodE).This synthetic approach could be explored to build heterotet-

ranuclear complexes since it allows the construction of thetetranuclear framework in two steps. If appropriate startingmaterials containing different metals are used (M and M′),tetranuclear complexes having a heterometallic core [M2M2′]could be prepared. Hence, when [Rh(acac)(cod)] is reacted with1 equiv of Py(SH)2 and then with a half-equivalent of thedinuclear complex [{Ir(µ-OMe)(cod)}2], a dark purple micro-crystalline solid is obtained. The mass spectrum shows a peakfor the expected heterotetranuclear core [Rh2Ir2] atm/z) 1306but also peaks atm/z ) 1396, 1218, and 1126, correspondingto the cores [RhIr3], [Rh3Ir], and [Rh4], respectively, areobserved. In order to elucidate if the observed distribution ofproducts arise from recombination of produced fragments inthe MS spectrum, a careful inspection of the1H NMR is

necessary. The aromatic region shows multiple doublets forthe most deshieldedmeta protons of the aromatic ring; thenumber of observed resonances is greater than that expectedfor the [Rh2Ir2] aggregate, even if the four possible isomers areaccounted for. So, the lack of selectivity in the synthesis ofthe heterotetranuclear aggregates is a direct consequence of theway of formation and probably of the nature of the binuclearprecursor8. Attempts to obtain separate fractions with identicalmetal core by recrysallization were unsuccessful.In a parallel manner, when [Ir(acac)(cod)] is reacted with 1

equiv of Py(SH)2 and then with a half-equivalent of [{Rh(µ-OMe)(cod)}2], solid samples containing heterotetranuclear ag-gregates with cores [Rh2Ir2], [RhIr3], and [Rh3Ir], but no [Ir4],are obtained. Monitoring the reaction by1H NMR shows somedegree of selectivity at the first stages since four doublets, forthemetaprotons are dominant over the others. Nevertheless,a distribution of products similar to that found in the aboveexperiment is observed when the reaction time is increased to24 h, suggesting that an intermolecular rearrangement of thetetranuclear frameworks formed at the beginning of the reactionoccurs.Electrochemical Studies.The cyclic voltammetry of [Rh4-

(µ-PyS2)2(cod)4] (1) in dichloromethane shows three successiveanodic processes at peaks A-C (Figure 6a). The analysis39,40

of the waves A/E and B/D with scan rates varying from 0.05 to0.20 V s-1 indicates that they are reversible one-electronoxidations.41 The third anodic response (peak C) at 1.21 V (at0.1 V s-1) shows typical features of an irreversible electrontransfer.40 The response of complex1 at the RDE confirmsthat the two first steps at the formal electrode potentialsE1/2 )0.16 V andE1/2 ) 0.58 V, corresponding to the waves A/E andB/D, are one-electron oxidations, which is in accordance withthe formation of the mono-[Rh4(µ-PyS2)2(cod)4]+ and dicationic[Rh4(µ-PyS2)2(cod)4]2+ species having the cores [Rh4]5+ (Z )+1) and [Rh4]6+ (Z ) +2), respectively. The third step,corresponding to the anodic process C, has a double limitingcurrent per concentration unit relative to the other two.Therefore, the anodic response C probably involves a two-electron process. The electrochemical behavior for the relatedcomplex [Rh4(µ-PyS2)2(tfbb)4] (3) in dichloromethane is quitesimilar to that found for1. Two reversible one-electronoxidations are observed at formal electrode potentials (E1/2) 0.37and 0.79 V. Interestingly, both are shifted 210 mV to higherpotential relative to those observed for1.Complex [Rh4(µ-PyS2)2(CO)8] (6) shows irreversible oxida-

tions at-0.05,+0.62, and+1.21 V (at 0.1 V s-1). The waveup to the solvent anodic limit gives poorly defined voltammetricpictures because of the coalescence of several oxidation steps.When the scan is reversed, two broad irreversible reductionpeaks at+0.2 and-1.38 V (at 0.1 V s-1) are observed.Analysis of the cathodic response indicates that both reduction

(39) Brown, E. R.; Sandifer, J. R. InPhysical Methods of Chemistry:Electrochemical Methods; Rossiter, B. W., Hamilton, J. F., Eds.;Wiley: New York, 1986; Vol. 2.

(40) Analysis of waves A/E and B/D shows values ofip(E)/ip(A) and ip(D)/ip(B) ratios equal to 1 at any scan rate; theip(A)V-1/2 andip(B)V-1/2 currentfunctions are constant, and the differences [Ep(A) - Ep(E)] and [Ep(B)- Ep(D)] are constantly equal to 63 and 72 mV, respectively. The∆Ep term departure from the theoretical value of 59 mV is rathercommon when dichloromethane is used as solvent (Mabbott, G. A.J.Chem. Educ. 1983, 60, 697). Analysis of peak C with scan ratesvarying from 0.01 to 1 V s-1 shows no reduction peak associatedwith C. In addition, the current functionip(C)-1/2 is approximatelyconstant, indicating that the process is diffusion-controlled and thevalue of the potentialEp(C) increases by 70 mV for a 10-fold increasein V.

(41) Piraino, P.; Bruno, G.; Tresoldi, G.; Lo Schiavo, S.; Zanello, P.Inorg.Chem. 1987, 26, 91.

Scheme 2. Synthetic Methods for [Rh4(µ-PyS2)2(cod)4]and [Rh2(µ-PyS2H)2(cod)2]


waves are associated to two different electrogenerated species.The location of the two main oxidation waves for complex6shifted 460 and 250 mV to higher potentials than those observedfor 1 and 3, respectively, is in agreement with the greaterπ-acidity of the carbonyl ligands when compared with tfbb andcod.Replacement of carbonyl ligands by less strongπ-acceptor

ligands as triphenylphosphine should increase the electronicdensity on the rhodium atoms, and the resulting complex shouldbe easier to oxidize and harder to reduce. As expected, thecyclic voltagram of [Rh4(µ-PyS2)2(CO)4(PPh3)4] (7) in dichlo-romethane shows anodic processes shifted negatively from thoseobserved for6. In fact, two well-defined peak systems areobserved at-0.06 and+0.36 V together with several irrevers-ible oxidations at+0.60, +0.83, and+1.02 (at 0.1 V s-1).Analysis of the two first waves with scan rates ranging from50 to 300 mV s-1 indicates that they are quasireversible electrontransfers complicated by the following chemical reaction.42

The cyclic voltagram of [Ir4(µ-PyS2)2(cod)4] (4) in dichlo-romethane is rather complex and involves five subsequent anodicprocesses (Figure 6b). Analysis of the main peak systems A/Cand B/D follows the reversibility criteria above described,40 andhence, the anodic processes atE1/2 ) 0.08 V andE1/2 ) 0.53 Vshould correspond to reversible one-electron oxidations assigned

to the formation of the mono-[Ir4(µ-PyS2)2(cod)4]+ and dica-tionic [Ir4(µ-PyS2)2(cod)4]2+ species having the cores [Ir4]5+ (Z) +1) and [Ir4]6+ (Z) +2), respectively. The anodic processesat +0.84,+0.99, and+1.30 V (at 0.1 V s-1) correspond toirreversible oxidations. On reversal of the scan, reduction wavesat +1.2,+0.73, and-0.37 V are observed, which arise fromthe reduction of previously oxidized species; in addition, thecathodic response shows also an irreversible reduction step at-1.12 V (not shown in Figure 6b).On sight of these electrochemical features, the rhodium and

iridium diolefinic complexes1, 3, and4 are the best candidatesto undertake the chemical synthesis of the mono- and dicationicspecies since they undergo two reversible oxidation processesat formal electrode potentials which are accessible by chemicalreagents. For each complex the two oxidation waves areseparated approximately by∆E° ) 0.4 V (∆E° ) differencebetween the formal potentials of the two one-electron-oxidationsteps), suggesting that the metals are communicating, sinceotherwise both oxidation waves should move closer togetheras has been found in dinuclear ferrocenyl derivatives.43 Withregard to the chemical synthesis, the monocationic speciesshould be stable with respect to disproportionation since a largewave separation is observed. According to the Gagne´’ssuggestions for dinuclear complexes44 if one considers thefollowing comproportionation equilibrium between tetranuclearcomplexes:

[M4] + [M4]2+ h 2 [M4]

+

the equilibrium constantKcomcan be estimated as a function of∆E°. In our complexes the obtained values areKcom ) 1.2×107 (1 and3) and 4.1× 107 (6). Although structural changesand several effects are reflected inKcom through changes in∆E°,the electronic delocalization of the positive charge over all fourmetal atoms in the monocation enhances its stability, resultingin higher∆E°. Large values ofKcom(1034) are due to the specialstability of the delocalized valence. When no interaction takesplace between the metals,Kcom ) 4 on the basis of statisticaldistribution of charges and results in a∆E° value of 0.036 Vand a single voltammetric wave.44 Similar values ofKcom tothose found for our complexes have been found for tetranuclearMo and Wo complexes5c and for the mixed-valence ion [(NH3)5-Ru(pyrazine)Ru(NH3)5]5+, which exhibits electronic delocal-ization; the reported Ru(III)-Ru(II) reduction potentials45 forRu(III)-Ru(III) f Ru(III)-Ru(II) f Ru(II)-Ru(II) gives∆E°) 0.39 andKcom ) 4 × 106. For our complexes extensivedelocalization of the charge between the metal centers occurs.Therefore, the removal of electrons should occur from thehighest occupied molecular orbital delocalized over the tetra-metallic core.

Acknowledgment. We thank DGICYT (Projects PB92 86-C02-02 and PB94-1186) and CICYT and Generalitat deCatalunya (Project QFN92-4311) for financial support andDiputacion General de Arago´n for a fellowship (to M.A.C.).

Supporting Information Available: Tables of anisotropic displace-ment parameters, atomic coordinates for hydrogen atoms, full experi-mental details of the X-ray studies (Tables S1-S3), bond distances,angles, and interatomic distances and figures of the full assignment ofthe protons of the COSY spectra (Figures 3 and 4) with overlayssuperimposed (23 pages). Ordering information is given in anymasthead page.

IC950849U

(42) Bianchini, C.; Frediani, P.; Laschi, F.; Meli, A.; Vizza, F.Inorg.Chem.1990, 29, 3402.

(43) Geiger, W. E.; Connelly, N. G.AdV. Organomet. Chem. 1985, 24,87.

(44) Gagne´, R. R.; Spiro, C. L.; Smith, T. J.; Hamann, C. A.; Thies, W.R.; Shiemke, A. K.J. Am. Chem. Soc. 1981, 103, 4073.

(45) (a) Elias, J. H.; Drago, S. R.Inorg. Chem. 1972, 11, 415. (b) Creutz,C.; Taube, H.J. Am. Chem. Soc. 1973, 95, 1086.

Figure 6. Cyclic voltammograms of 5× 10-4 M solutions of (a) [Rh4-(µ-PyS2)2(cod)4] (1) and (b) [Ir4(µ-PyS2)2(cod)4] (4) in CH2Cl2/0.1 MBu4NPF6 at the scan rate 0.1 V s-1.


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